Should you buy a diesel or a gasoline truck to tow your travel trailer or fifth wheel? What are the advantages of diesel engines, if any? I see this question frequently on social media and have these answers.

It used to be pretty simple to determine whether gas or diesel was better but times they have changed. There are a lot of things you might take into account today when considering which fuel choice is better for your towing situation starting with what you are towing.

The numbers - it’s always the numbers

The first thing to consider when choosing between diesel or gasoline is your numbers. While many buyers focus on the pulling capacity of a vehicle, what is typically the limiting factor is the carrying capacity of your tow vehicle.

Your tow vehicle typically carries about 15 percent of the trailer’s total weight for a bumper-pull or travel trailer. You also have to factor in what’s in the vehicle you’re towing with including passengers, cargo, pets, firewood, generators, your mother in law and everything else.

We have a RV towing calculator you can use here.

When you read the trailer towing numbers that many vehicle manufacturers publish know that this is figured by towing a very unique sled that has low tongue weight and almost no wind resistance. Both of these are very different than what you’ll experience towing a travel trailer or fifth wheel.

Why choose a diesel engine

For generations the diesel engine has been the choice for vehicles that haul heavy loads in the US. Around the world up to half the passenger vehicles are powered by diesel engines as well. Why?

A diesel engine works by compressing the fuel/air mxiture so much that it explodes. A gasoline engine compresses the fuel/air mixture as well but not as much and needs a spark plug to explode the mixture to create power.

Because a diesel engine is a compression ignition engine, it has to be built to much higher tolerances. As such diesel engines tend to last much longer than gasoline engines. They’re just tougher because they have to be.

The nature of this compression also makes diesel engines more efficient. Longer life and better fuel economy mean that commercial users, and passenger car drivers in Europe, favor diesel engines for their efficiency.

But the down side is that diesel engines have produced more emissions than their gasoline counterparts. As such regulations are becoming more and more stringent on diesel engines and they have gone from being simple, reliable sources of power to rather complicated systems with intricate emissions technology that has to be maintained.

These complicated emissions systems have made diesels a less clear choice even for heavy hauling and vehicle manufacturers are catching on to this. For example, Ford’s recent “Godzilla” 7.3 liter gasoline engine is a huge gasser that is meant to provide much of the towing capacity that people had traditionally looked to diesel trucks to provide.

The core diesel engine is likely as reliable and stalwart as ever, but the long-term reliability of these complicated and very expensive emissions systems is still being determined.

Does better fuel mileage make sense?

By the nature of diesel fuel and the combustion process, a diesel engine has been more fuel efficient than it’s gasoline counterpart. Further, the design of a diesel engine typically means that there’s less horsepower but more torque, the twisting force, and that’s what you want when pulling a heavy load.

However the cost to produce a diesel engine is higher so the logic of buying a diesel engine strictly for fuel economy may not make sense. For example, the additional cost of a diesel engine in many heavy-duty trucks can be around $10,000. It would take a tremendous amount of driving to zero-out that big of an up charge if you’re choosing a diesel engine on the basis of fuel mileage alone.

Further, in most places, diesel fuel is more expensive than gasoline since taxes are significantly higher on diesel fuel. What was once almost a give-away waste product has now become a premium item and I have seen up to $1.50 a gallon more for diesel fuel, although that varies greatly on where you’re buying it.

Further, modern diesels now require diesel exhaust fluid, or DEF. While not a huge expense, it is one that has to be factored in.

Diesel maintenance

Another thing to factor in is the cost of maintaining any engine. Gasoline engines have become very, very reliable with much less need for scheduled maintenance. Aside from routine maintenance including oil changes and filters, there isn’t much to do on a modern gasoline engine.

Diesels, too, can require little maintenance and they don’t even have spark plugs! But know that the heavier build of a diesel means they also have more oil so oil changes can be significantly more expensive. Further, make sure you have your maintenance lined-up. Some places only work on gasoline engines and finding a diesel mechanic for some engines in small towns might be a tough call.

Diesels tow more, right?

The real reason for a diesel engine is towing, right? Well, maybe.

In many cases the diesel engine option is only available on the most capable variants of a full-sized pickup truck. But know that the heavier build of a diesel engine also means that the engine itself weighs more. That can take away from the cargo carrying capacity of a truck since some of that capability is simply used for hauling around that big engine.

This can absolutely come into play in some fifth wheel applications where you’ve got a lot of pin weight to consider. Know the capability of any specific truck before making a buying decision.

Should I buy a gas or diesel engine?

There is no single answer to this. A lot of the decision has to be based on what you’re towing now and what you hope to tow in the future.

There’s no denying that the torque of a diesel engine plus the sound of a diesel engine appeals to people including myself. But the higher cost of a diesel engine including maintenance means you have to absolutely have a need for the additional torque or just want the darned thing.

Further, diesel fuel isn’t as commonly sold as gasoline with every podunk city having severe choices for gasoline but not necessarily diesel fuel. I like traveling to podunk little towns - they’re interesting.

There have also been recent cases where some diesel engine emissions systems have had issues significant enough to shut the engines down, leaving some RVers stranded in the middle of nowhere.

While a diesel engine will deliver better fuel economy that difference may not be enough to justify its additional cost. If fuel costs are your only consideration, it’s difficult to justify on this basis only unless you drive a lot of miles.

Some of the advantages of a diesel engine in the past, including exhaust braking, are minimized by newer automatic transmissions that use creative gear choices to accomplish similar things to exhaust braking. I know the eight-speed automatic in our own Ram truck will automatically downshift going down hill when hauling our trailer to the point that I often don’t have to even touch the brake pedal on mountain descents.

My conclusions

My conclusion is this - I would favor the gasoline engine in many cases because it’s going to cost less to purchase and less to maintain over time. Unless you drive a tremendous amount it will be difficult to justify the additional cost of a diesel engine from a financial standpoint alone.

There are now larger gasoline engines available once again that may have more than enough capability for what you are towing. Some of the big gasoline V8 engines now have technology that can shut-down four of the eight cylinders so you’re operating more efficiently under light loads, such as if you use the pickup truck as a commuter vehicle.

Diesel is no longer a clear winner for those of us who aren’t commercial truck drivers. With added cost of fuel, maintenance and just the sticker price of the diesel engine option there are a lot of reasons a gas engine may suit your needs just fine.

What do I know

A lot of people have opinions on this front but my background in this is that my dad was instrumental in developing the Garrett T04E turbocharger. This device has really changed the face of diesel engines. Growing up engines, particularly diesels, were a part of my life and I got to tag along for lab and field testing and was explained the inner workings of diesel engines.

I love all things mechanical and have written a nationally-syndicated column about vehicles.